1. Field of the Invention
The present invention relates to the novel use of stilbene compounds to effectively enhance the toxicity of one or more chemical pesticides. The invention further relates to new compositions containing stilbene compounds and the chemical pesticide(s).
2. Description of the Related Art
Although several commercial formulations of entomopathogens (e.g., viruses and bacteria) have been available for many years, their use in agricultural and forestry pest management programs has been rather limited when compared to use of conventional insecticides. In general, efficacy of currently available formulations of entomopathogens has been considered to be inferior to that of conventional insecticides. Two factors commonly associated with inadequate efficacy of entomopathogens against pestiferous insects are (1) length of time required to subdue the pest (i.e., unacceptable level of crop-damage may occur before the pest succumbs to factors associated with pathogenic infection) and (2) short residual effectiveness against pest infestation (e.g., stability of entomopathogenic viruses is adversely affected by exposure to ultraviolet solar radiation; Jaques, Can. J. Microbiol., 14: 1161-1163 (1968)).
Doan et al. (J. Insect Pathol., 6: 423-429 (1984)) suggested that greater insect control could occur with the use of viruses if the formulation could be enhanced by the addition of certain adjuvants. Potential virus adjuvants included selected analogs or salts of the stilbene compound, 4,4'-diamino-2,2'-stilbene disulfonic acid. Two such analogs are Calcofluor White and Phorwite (Shapiro et al., U.S. Pat. NO. 5,124,149. Results from several laboratory studies reported by Shapiro et al. demonstrated that Calcofluor White M2R potentiated (as much as 1000-fold) the virulence of the nuclearpolyhedrosis virus (NPV) against the gypsy moth larvae, Lymantria dispar (as determined by enhanced LT.sub.50 and LD.sub.50 values). Shapiro et al., however, made no claims or suggestions regarding the use of stilbene compounds as enhancers of chemical pesticides (i.e., nonentomopathogens).
Although the mode-of-action pertaining to the stilbene's ability to potentiate the virulence of a virus against an insect is still speculative, there are reports in the literature regarding the biological properties of stilbene compounds. Stilbenes, and at least some of their photoproducts, have been shown to irreversibly bind to proteins in wool, silk, bovine serum albumin and apomyoglobin (Holt et al., Aust. J. Biol. Sci., 27: 23-29 and 195 (1974)). It has been demonstrated that certain stilbene compounds can inhibit cellulose and chitin microfibril formation (Roberts et al., J. Cell Biology, 9: 115a (1981); Herth, J. Cell Biology, 87: 442-450 (1980)). The stilbene, Calcofluor White, prevented formation of cellulose microfibrils in Acetobacter xylinum by hydrogen bonding with glucan chains (Haigler et al., Science, 210: 903-906 (1980)) and inhibited chitin synthetase activity in Neurospora crassa (Selitrennikoff, Exp. Mycol., 8: 269-272 (1984)). Shapiro et al. found that another stilbene compound, Phorwite AR, synergized a cytoplasmic polyhedrosis virus (CPV) against gypsy moth larvae. Since CPV multiplies only in the midgut epithelial cells, it has been suggested that the site of action of the brightener was the insect's midgut (Dougherty et al., "Mode of Action of Fluorescent Brighteners as Enhancers for the Lymantria dispar Nuclearpolyhedrosis Virus (Ld NPV) in the Gypsy Moth," oral presentation, Am. Society for Virology, Colorado State University, Fort Collins, Colo., Jul. 9, 1991). With the exception of some fluid-feeding species, many insects possess a midgut which is lined with peritrophic membrane, which in turn, is comprised of chitin and protein (Chapman, The Insects Structure and Function, p. 46 (Elsevier, N.Y. 1971)).
By way of additional background, U.S. Pat. No. 3,878,201 discloses certain pentadien-3-one substituted amidino hydrazones and the salts thereof. U.S. Pat. No. 4,087,525 describes the insecticidal use of these pentadienone hydrazone compounds. However, in practice, a representative hydrazone compound, hydramethylnon, has been shown to be a relatively poor contact insecticide and fair stomach insecticide against lepidopterous larvae (Hollingshaus et al., Pesticide Biochem. and Physiol., 22: 329-336 (1984)). In their 1984 study, Hollingshaus et al. found that when treated foliage was fed to larvae of tobacco budworm, Heliothis virescens, only around four to eight percent of the initial dose of hydramethylnon ever absorbed through the gut and into the hemolymph during the first 72 hours posttreatment. Most of the hydramethylnon was adsorbed to gut contents and tissue or excreted in the feces. If action sites for hydramethylnon exist outside of the insect's gut, the aforementioned data would suggest that hydramethylnon is a fairly inefficient toxin (since only 4% to 8% of the entire dose would ever reach those action sites). Other pesticides having similar activity like the pentadienone hydrazones include the insecticide rotenone as well as the insecticidal and acaricidal cinnamamide compounds. The cinnamamides are described in U.S. Pat. Nos. 4,659,857 and 4,775,693. As a consequence of the inefficiency of these pesticidal compounds in practical use, it is highly desirable to find a means for enhancing the absorption of the active ingredient through a lepidopteran's gut and thus improve or hasten movement of the toxin to action sites.